Reply only to the necessary people (don't abuse CC: or "reply to all").

Be short and concise. Don't ramble (stay on topic).

Break up your paragraphs.

Use proper spelling, grammar and punctuation (avoid CAPS).

Don't answer spam, and don't send out spam.

Break up your paragraphs.
When composing a message, creating paragraphs of text is critical to readability. You'll notice I've done that with this post here. After four or five sentences, and especially when I create a new topic or idea under the topic, I start a new paragraph. I do so by giving an extra line break between the two paragraphs, so your eye can more easily move from one to the next, and stay on the sentence without getting lost. Do this in your email.

When there are no paragraphs, reading the email can be daunting. Especially if the text is one long, and very big paragraph. Because paragraphs are meant to break up ideas and topics, take advantage of it. As with other points brought up, it's courteous to your reader. Also, it will force you, somewhat, to evaluate the length of your email, and identify whether or not you have been short and sweet. After all, the more paragraphs you have in your message, the longer your email is getting.

Use proper spelling, grammar and punctuation (avoid CAPS).
On thing email is not, is SMS text messaging. It's not even writing by hand, as most typists can type faster than they can write. So, it is not acceptable to use "u" for "you", "ur" for "your", "thx" for "thanks", etc. You have the keys on your keyboard for a reason- use them. Further, using this "txt spk" makes it difficult to read for many. Email isn't confined to a limit on the number of characters you can send per message, and you generally aren't charged per email that you send. So there is no reason to shorten your speech using this method.

You also have a shift key and all the necessary punctuation keys on your keyboard. Use those as well. This means use uppercase characters when you should. Use punctuation appropriately (EG: "Let's eat grandpa!" versus "Let's eat, grandpa!"). Most email clients come with built-in spell checkers. Use them. Nothing shows a lack of intelligence more than a terrible speller. Not only spelling, but grammar too. "You're" versus "Your". "It's" versus "Its". "They're" versus "Their" versus "There". "Than" versus "Then". Et cetera.

LASTLY, AVOID USING UPPERCASE WHEN COMPOSING YOUR EMAIL. See how that seems almost like I'm shouting? Using CAPS is appropriate in certain situations, but as with everything, it should be used sparingly. When you compose your entire message in CAPS, you come across as angry and shouting. Immediately, people will become defensive, and you will probably get replies showing such.

Don't answer spam, and don't send out spam.
Unfortunately, one of the thorns in our side, is to deal with spam email from mass advertising, marketers, scammers and phishers. Don't reply to these messages. It only adds fuel to the fire, especially if the spam is on a mailing list. Just ignore it, and move on.

Also, don't send out spam. I learned this the hard way. When Firefox reached a 1.0 milestone (I had been using it around the 0.8 days), I sent an email about its release to everyone in my contact list. Family, friends, school teachers, church acquaintances, random people I had only met once, etc. The message probably went out to 200-300 people. It wasn't long before I got a lot of replies from some frustrated people who were already familiar with the release, and didn't appreciate me sending them an email about it. In fact, my email was referred to as "spam". I was a little taken back. I wasn't spamming at all. At least, I wasn't trying to illicit information or make a sale by any means. But then, I realized that my mail was unsolicited, and there could have been a better means of getting the information out, if I really wanted them to know about it.

The same is said in general. There are times and places for announcements, and they are certainly appropriate in the right list or under the right occasion. It's the ability to identify that time and place. If you're unsure, then you should probably avoid it.